With an increase in capacity of information processing in recent years, various information recording technologies have been developed. In particular, the surface recording density of an HDD using magnetic recording technology is continuously increasing at an annual rate of approximately 100%. In recent years, an information recording capacity exceeding 160 GB per one magnetic disk with a 2.5-inch diameter for use in an HDD or the like has been desired. To fulfill such demands, an information recording density exceeding 250 Gbits per one square inch is desired to be achieved.
To attain a high recording density in a magnetic disk for use in an HDD or the like, a magnetic disk of a perpendicular magnetic recording type has been suggested in recent years. In a conventional in-plane magnetic recording type, the axis of easy magnetization of a magnetic recording layer is oriented in a plane direction of a base surface. In the perpendicular magnetic recording type, by contrast, the axis of easy magnetization is adjusted so as to be oriented in a direction perpendicular to the base surface.
For example, Patent Document 1 discloses a technology regarding a perpendicular magnetic recording medium configured to have a ground layer, a Co-type perpendicular magnetic recording layer, and a protective layer in this order formed on a substrate. Also, Patent Document 2 discloses a perpendicular magnetic recording medium having a structure attached with an artificial lattice film continuous layer (exchange coupling layer) exchange-coupled to a particulate recording layer.
In the perpendicular magnetic recording type, compared with the in-plane recording type, a thermal fluctuation phenomenon can be suppressed, and therefore the perpendicular magnetic recording type is suitable for increasing the recording density.
Also, conventionally, for the purpose of improving a coercive force Hc and a Signal-Noise Ratio (SNR), a structure in which a magnetic recording layer is formed of two layers has been suggested. For example, Patent Document 3 discloses a configuration in which, with a magnetic recording layer being formed of two layers, one recording layer on a base side has a composition containing CoCrPtTa and the other recording layer on a side away from the side has a composition containing CoCrPtB.    Patent Document 1: Japanese Unexamined Patent Application Publication No. 2002-92865    Patent Document 2: U.S. Pat. No. 6,468,670    Patent Document 3: Japanese Unexamined Patent Application Publication No. 2001-256632